C.G. issue, Ambulance AS350

[ron-powell]

Guys and Gals:

I need some deep end of the pool technical help here.

Our Astar has a stretcher system which allows fore and aft movement. When loaded in the aircraft, the patient stretcher locks into an articulating tray. The tray can move laterally via the floor mounting pedestal and fore and aft via locking rollers on the pedestal. This articulation facilitates loading and unloading. The mounting point of the pedestal, which the stretcher/tray is attached to, has no fore/aft movement, only lateral. Aircraft C.G. is computed using this arm position.

We occasionally run into a heavy patient, driving the C.G. out the front end and therefore we decline the flight for controllability reasons. A conservative maximum patient weight for the daily crew mix is computed based on the C.G using the aforementioned arm position of where the stretcher is attached to the floor and using the maximum gross weight (since we operate near it all the time) forward C.G. limit.

The question is, if this heavy patient is moved as far aft as possible on the stretcher/pedestal, maximum fore/aft movement being on the order of 4-6 inches, does this affect C.G. to the point we could possibly carry this heavy patient. And if so, is there some formula (hopefully something I can use on a spreadsheet) that will mathematically show this?

The background behind this is the discussion that moving the stretcher aft allows for a heavier patient to be carried because the patient weight moves aft even though the stretcher has a fixed fore/aft mount. The other side of the discussion being there can be no C.G. change since the stretcher, regardless of weight as it affects the stretcher mount, only affects C.G. at the mount point on the floor.

Furthermore, there is no accommodation for C.G. shift in the stretcher paperwork/manual I can find that would explain any possible shift of C.G. via the stretcher fore/aft positioning.

[albatross]

Good question and way beyond me but why not contact the manufacturer of the stretcher installation.
I think that basic physics is involved and something to do with levers and forces is a clue. The single point mount is the issue. If only I had paid attention in grade 9 physics.
The load distubution on a stretcher is never symmetrical VS stretcher center anyhow ( hence why I always go for the feet end when carrying is called for LOL ) so you have another variable there too.
Sounds like you have some big ones in your end of the world.
In any case I am sure some smart person will be along with an answer soon. It may even be correct.

[SASless]

Ron,

I would opt for a bigger helicopter....say a 412...but what an interesting question you pose!

Knowing most flight nurses fill out a flight suit to the max limit of the tensile strength of the kevlar stitches.....perhaps a field trial or two might prove useful.....assuming you can get one of your flight nurses to fit onto the stretcher.

[What Limits]

You may find that the manufacturer has set a 'Null' position for the articulating stretcher and the STC is based on this. The helicopter CG is also based on this thus giving you very little scope to move the strethcher fore and aft without affecting airworthiness.

[Pilot DAR]

Not knowing the details of you arrangement, I cannot offer a detailed comment on your specific question, however, consider some of the overall aspects of what you're working with:

Weight and balance information is clearly spelled out in the flight manual for the helicopter, and very easy to work out. As long as you know the weight of the stretcher assembly, and the weight of the proposed patient, you can do the math. That will tell you what the helicopter is capable of from the flight standpoint. Do not exceed the weight and balance limtations, though operation just within the edges will be legal and safe.

Next you must consider what the limitations of the installation are from the standpoint of its approval (probably STC). What do they ( the manufacturer or approval (STC) holder) say you can do? If what you propose to do exceeds one of those limitations, it will require further approval, which under some circumstances is possible ( I've done it before).

Ask youself: Is the structure and design of the stretcher intended to carry the required crash loads in the other possible positions? Perhaps one of the positions has greater locking strength for securing than other positions. You won't be able to answer "yes" to this question, but if you see an obvious "no", you should probably not goo too much further.

How about the floor attachments in the airframe. They are obviosly adeqate for the approved arrangement. If you use the stretcher differently, will the attachment of the stretcher to the floor impose loads on the floor for which desgin compliance has not been shown? Just because something, or some way of doing something with an aircraft is not approved, does not mean that it cannot be, but there's a process which must be followed. As before, you can't give a yes, but might realize why the no once you look more closely.

If you need more detailed information on this, feel free to PM me. In the mean time, be very sure that the stretcher is installed and used as intended and approved. If in doubt, reread the flight manual supplement and installation instructions. Failure to use the stretcher as stated in those publications puts the pilot and operator far out on a legal limb - don't go there without approval.

Pilot DAR

[john_tullamarine]

Considerations (which largely are independent of the installation) -

(a) both long and lat CG are important for helos

(b) the CG of the patient's body, when recumbent, needs some care as body shape may have an input although I would guess not to a really great extent. I have never looked at this particular matter but it can be resolved fairly easily either by

(i) a statistical look at a range of body shapes ie weigh and determine the CG for a sample recumbent population to come up with some guidance material for operational use so far as CG is concerned

(ii) having several aircraft kit bathroom scales so that you can weigh the patient, recumbent on the stretcher, prior to loading. It is a trivial exercise then to generate a process to determine both patient weight and CG for the helo loading calculations. The bathroom scales would be fine so long as they are reasonably repeatable and are calibrated periodically.

(c) Aircraft C.G. is computed using this arm position

Without your paperwork in front of me I can't comment on your specific loading system. However, the installation weight/CG is straightforward - the patient CG is not ergo you need a system which can account for patient CG variation in the recumbent position - I am presuming that you weigh the patient somewhere along the way (no pun intended) - and also for any intentional variation you might generate by shifting the stretcher on the mounting assy

(d) if I read the posts correctly, the specific location of the patient's body will affect the aircraft loaded CG very definitely ... the loads carried by the attachments will vary according to the position of the patient and are then fed into the ship's floor/wall as appropriate.

(e) Before you shift the patient on the mounting assy .. are the load restraint provisions compromised ? ie is the design of the installation predicated on the stretcher's being in one or other specific positions for restraint considerations ?

(f) how constrained are you from a point of patient harness restraint system ?

(g) easiest fix is to dial up your local DER and get him/her to run with the problem through to approved solution. Which ever way you look at it the problem

(i) should be fairly easy to define/fix

(ii) potentially is sufficiently of a worry that you should fix it rather than ignore it.

(I see that Pilot DAR and I have posted concurrently .. between the two posts you have a reasonably comprehensive story telling ...)

[KrisRamJ]

I agree with the previous post - moving the patient aft could pose extreme loads on the mount point for the stretcher or the location on the aircraft to which the stretcher attaches. It's worth checking that documentation before even thinking about anything else.

That said, moving the patient aft will definitely move the helicopter's CG aft. The reason for this is because the single mount point for the stretcher generates both downward and upward forces that effectively move the aircraft's CG depending on the position of the patient in relation to the mount point. I'm surprised this isn't mentioned in the documentation for the stretcher to be honest - your CG calculations should be figured out for the center of mass of the stretcher & patient combination, NOT for its attachment point to the floor. Perhaps doing this will allow you to carry heavier patients straight away?

Another way to think about this is those huge steadycam mounts you see on the left side of AStars - if CG was only dependent on the position it connects to the aircraft then you could carry that camera way out on a long arm and it wouldn't matter. Of course arm length does matter, because it imposes a couple that 'pulls' CG in a lateral direction. It's more tricky to visualize when it's happening inside the aircraft but the same principle is at work.

If you still have problems with loading then a good formula to use for weight shift is D/d = W/w where:

D = distance weight is shifted
d = change in CG position
W = total weight of aircraft
w = weight shifted

For example, say you want to move the CG 0.2 inches back to get the helicopter into the forward CG envelope, the patient weighs 200lb and the total weight of the aircraft is 5000lbs then the distance the patient must be moved is: 5000/200 x 0.2 = 5 inches.

An important consideration of course with the AStar is that as fuel burns off then the CG will move forwards.

Hope that helps

Kris

[Gomer Pylot]

I don't think you can get by with moving the patient, because I don't think you can insure that the patient's CG will be at any location. The patient can move himself a few inches one way or another in flight, unless fully packaged, so you have to be a little conservative, I think. You can try doing a weight/balance by hand using both arms and see what you get, but I don't think the CG will change by much. You pretty much have to compute the wt/bal using whatever parameters the manufacturer and the STC give you. You don't want to get caught fudging that. In reality, the helicopter will still be controllable even a little outside the limits, because the manufacturer has to insure that it is fully controllable at the limits. Legality requires staying inside the limits, though. If you have to turn down too many flights, your employer could consider sending some larger aircraft from the GOM. Expect that soon. :rolleyes:

[ThomasTheTankEngine]

Hi If all else fails you could fit a tail cone weight, Ive seen these fitted to compensate for nose mounted camera systems that would bring the CG to far forward. This would of course increase the empty weight of the helicopter but not to much.

[gomez308]

We added 15 lbs to the tail cone on our new B2. All the Astars I've flown have had some amount of weight added back there.

[victor papa]

After the initial fitment of our medical kit(also swivelling load system), we weigh our machines and add weight in the tail so she is as far rear C of G empty as is allowed We have had to fly some on the limit patients(136kg) regularly and between 6 350's never had a report of forward c of G problems inflight or on the weighing sheet once plotted. The additional weight is either 1kg/5kg units and if memory serves 25kg or something is max so it is not a train smash on MAUW.

[Jack Carson]

Pilot DAR is correct. You need to understand all the limitations of the medical interior as installed. First and foremost the RFM’s supplement section includes the published limitations of the litter installation. There may also be a placard located near the litter on the floor. Typically, there is only one certificated location for the litter installation. This is a certified position. All structural testing is based on this position. Other positions may appear to work but would not guarantee the same level of safety should the aircraft be involved in a mishap. As a side note, all EMS A Stars that I have had experience with have required additional ballast in the tail to improve the operational capability for heavier patients

[ron-powell]

First thanks for the insights and thought provoking secondary questions.

This is mostly an academic discussion because we really do not run into this problem that often. More of a situation to plan for something out of the ordinary. My understanding is a crew was told a patient weight in pounds only to be informed later, while enroute, it was kilos. The pilot was just trying to figure how to get the job done.

Personally, having equipment that will allow a physical shift in C.G. without the ability to account for it has clouded my thinking here. Even though I understand how it works I am unable to see and adjust my numbers for it with any reasonable expectation of accuracy.

>SAS: Knowing most flight nurses fill out a flight suit to the max limit of the tensile strength of the kevlar stitches.....

Depending on the shift crew/pilot mix, the max patient weight for C.G. can range from as little as 236# to upwards of 380#.

>What Limits: You may find that the manufacturer has set a 'Null' position for the articulating stretcher and the STC is based on this. The helicopter CG is also based on this thus giving you very little scope to move the strethcher fore and aft without affecting airworthiness

I suppose the thing we have going for us is that the stretcher will only lock into place so far forward on the tray mechanism. The only reason to allow it to lock further aft would be the patient is tall and their feet will not have clearance up under nose. Having said that, a patient’s lower to mid torso, where you might assume their C.G. is near, is never in front of the pedestal floor mount point - the arm point where the weight and balance is calculated. Of course, there’s no accurate accounting for someone with a huge barrel chest, Dunlop tired midsection or “thunder thighs” and arse to go along with them.

BTW, the mounting point looks to be where the left hand seat would mount. That arm is at about 60 inches and the pilot’s eat is around 61.

Regarding the strength of the system, the stretcher itself has no weight limitation. When the tray mechanism is rotated outward to slide the stretcher into place, we limit that to 350# due to the possibility of crew back injuries and usually running into the forward C.G. limit anyway. Patient securing straps are a foot anti-submarining bag, ankle, waist and chest with shoulder straps. All are used on every patient unless something interferes like a backboard. The patient is strapped to that as well.

>john_tullamarine: (ii) having several aircraft kit bathroom scales so that you can weigh the patient, recumbent on the stretcher,……

John this would be the best solution although with operational and paperwork issues. Hospitals have scales for patients and we could use two of them, nose and tail on the stretcher. This would certainly allow us an accurate recumbent “patient” C.G. number to work with. On the paperwork though, the STC holder would probably have to generate a chart or formula for us to use with the patient C.G. number and position of the stretcher in the aircraft. That would be the most accurate answer. Carrying more weight though, scales, in these Astars is definitely a problem.

While we do get fairly accurate weights from a hospital when transferring their patients, unless they transpose pounds for kilos of course, scene flights are the basic guess. The W&B is then calculated based on that guess.

>Gomer P: I don't think you can get by with moving the patient, because I don't think you can insure that the patient's CG will be at any location.

Yeah, that’s the rub here. We have no way of knowing, physically or legally, where it actually is by allowing the stretcher to move fore and aft. As above, limited forward positioning and some aft movement will always work in our favor though.

To everyone who posted after Gomer and before this one:

We have the STC battery relocation in front of the horizontal stab and max lead in the tailcone.

Victor Pop: I use a spreadsheet and figured out a conservative formula for a maximum patient weight based on that day's crew mix. Since we are near gross weight most of the time, the formula only reflects a C.G. shift using that weight. I plotted out weights in increments of 10 pounds on the patient C.G. station and found a forward shift of between .14-.15 inches per 10 pound increase. Then subtracting actual C.G. from the Astar's gross weight forward C.G., the formula predicts a conservative max patient weight. Now if I do an actual W&B at some slightly lower gross, the max patient weight can be slightly higher. But I go with the more conservative number mainly because I'm older and want a little more margin................

You want to look at the spreadsheet, drop me a PM. ron

[Devil 49]

Ron-powell-
If you check the RFM, you'll find Eurocopter's EMS supplement has a standard, approved stations for patients, 69.68" in English units for the lower stretcher. Just as with the pilot's station at 61.02", one has no way of legally adjusting for full forward or full aft location. The book numbers are the book numbers, amen, and you have to use them unless you have something else officially blessed.
The loading system's mount point is a mechanical consideration and a limiting factor in some installations, but the patient's head's going to be in pretty much the same place regardless of stretcher type so that the medical folks can do that medical magic- thus the standardized numbers in the book.

Don't give up on the substantial citizen/prospective patient without considering the movement of some weight aft. Junk-er- I mean "important medical equipment" in the cabin that can be moved into the aft hold is a big help. My experience is that one's standard load is gear for any eventuality, lots of which won't be needed with any one patient. Run that by the medical staff, they'd probably appreciate the excuse to gain some room.
All the "important equipment" my employer, and probably yours, too, insists I take, usually in the side holds, can be moved aft as well. Ops manuals, tie downs, survival kit, lunch or whatever, it all helps, not as much as getting stuff out of the cabin...
The limiting factor, in my experience, is aft baggage volume, but remember that that bay has weight limits. I can generally get another 30-50 lbs inside the forward CG by packing the littlest hold full. 400+ pounders generally won't fit lateral or longitudinal, so their case is moot.

Final thoughts-
I calculate my max patient weight at 4961 lbs in a B2 (and I'd guess a B3, but I've never flown one). It'lll get really close with fuel burn, but I've never had the CG go out the front, calculated down to zero fuel, if I stay inside that limit. This is easy figure, even with paper and pencil:
All up weight- no patient, crew, junk, etc, no fuel, note weight and total moment;
Fuel weight that brings you to 4961 times fuel arm, add to AUW moment and note total;
Subtract the above fueled moment from 627,070.4 (4961 x 126.4);
Divide the difference by the difference between the fuel station and stretcher station;
You have max patient weight at max gross.
You can get a wider CG envelope at lesser TOW in those aircraft, but you have to calculate a landing CG as well.
If I'm in something more humble, like B or BA, I calculate max patient at zero fuel, a little more involved, so I use a spreadsheet.

[john_tullamarine]

Personally, having equipment that will allow a physical shift in C.G. without the ability to account for it

not to worry .. any half competent design (wo)man can sort that out for you .. providing that the structural bits are OK, the loading stuff is just a matter of tweaking the loading system to suit the need.

the stretcher itself has no weight limitation

most unlikely .. keep in mind that the design structure has to address the crash loads in the design standard (for the AS350 FAR 27.561 (http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr;sid=4912b213a324d3a81c81289d197b7312;rgn=div8;vie w=text;node=14%3A1.0.1.3.13.3.252.33;idno=14;cc=ecfr) will be the first stop to look - as always, this is the current static rule and may not apply explicitly to the 350).

although with operational and paperwork issues

simpler than you might think .. but probably/possibly not warranted if you ran a population CG distribution exercise

How you get around the loading system implications is a matter for the local regulatory jurisdiction but, from an engineering viewpoint, will be pretty trivial

For the lb/kg problem .. a Mk I eyeball of the patient on arrival will resolve that question

Just as with the pilot's station at 61.02", one has no way of legally adjusting for full forward or full aft location

while I can't comment on the specifics in a particular jurisdiction, this comment probably is not correct ... the loading system needs to address the regulatory requirements and, generally, can operate within the intent of the FMS even if there be minor variations for a specific application .. hence unless you have something else officially blessed


Useful to keep in mind that there is no basis for calculating loading to the gram and millimetre ... the empty weight and other data doesn't support such accuracy.

[rotorbrent]

A tailcone weight kit will bring her back into cg for you pretty easy for a heavy patient and keep you inside cg when you do not have patient. It is a couple of dollars from Eurocopter for the tailcone weight kit. And very common on EMS Astars to have or need the kit to keep having such a forward CG.

Brent

[arismount]

Carry more fuel than you need unless it causes you to overgross. Of course it probably will. But just in case it won't, the extra fuel will take your CG aft. The trick is, compute your landing condition with enough fuel to keep your CG within limits. In other words, you will have to land with more fuel than your reserve...in such cases, the extra fuel is ballast and you cannot use it. If you understand what I mean.

The other guy, Kris I think, gave you a real useful formula:

CG Shift/Distance Moved = Weight/Gross

In other words, with a 4800# GW and a 200 lb patient, shifting that weight 6 inches would move the CG 0.25" in the direction you move the patient. But the other guys are right, you are stuck with the arm given for patient centroid.

Best solution is to move all med gear as far aft as possible. Ask you maintenance to look into ballast weights in the tail as well. 10 extra lbs back there will move the CG well aft and should solve your problem for good.

Last but not least of course I know you would never ever fly out of CG intentionally but if you ever have reason to believe you are close to your forward CG limit, do not under any circumstances land the aircraft except facing into the wind. If you know what I mean.

[Winnie]

C of G does not change with fuel in the astar?

Thought the arm for the fuel was right under the mast.

Mind you I have only flown the Twinstar so perhaps I should keep my trap shut...

[ron-powell]

>D49: If you check the RFM, you'll find Eurocopter's EMS supplement has a standard, approved stations for patients, 69.68" in English units for the lower stretcher.

Hey, getting any rain?

The Lifeport system uses 60.5’ for the patient station. 69.68’ would be deluxe…….

>Don't give up on the substantial citizen/prospective patient without considering the movement of some weight aft. Junk-er- I mean "important medical equipment" in the cabin that can be moved into the aft hold is a big help.

Our trunk is seriously full. The Lifeport system takes both left and right baggage compartments, so no help there either.

Just so you know what I’m working with here, our empty weight is 3474#.

>John_T: Personally, having equipment that will allow a physical shift in C.G. without the ability to account for it
not to worry .. any half competent design (wo)man can sort that out for you

Yes, anything is possible, but it appears Lifeport is able to get this system certified with the simplicity of one arm station even though the stretcher has C.G. altering fore/aft movement.

>the stretcher itself has no weight limitation
most unlikely ..

The stretcher itself has no limitation. If the patient can be strapped to it fine. But the “system” weight limitation re crash loads comes into play when you place the stretcher in the aircraft. And as a correction to an earlier comment, the system max patient weight is 300#, per the RFM Lifeport supplement.

>For the lb/kg problem .. a Mk I eyeball of the patient on arrival will resolve that question

Yeah but you have to fly all the way someplace to find that out? When a flight request comes our way, originating from a hospital, if the patient weight isn’t provided, it’s the first question I ask simply because they have means of determining the patient’s weight. Everything else is secondary. I don’t care where we’re going or how good/bad the weather is. If we can’t carry the patient due to C.G. issues, it’s a non-starter and I go back to my nap. Now on a scene response, we go regardless and figure out how we’ll do it when we get there.

>rotorbrent: A tailcone weight kit will bring her back into cg for you pretty easy

We apparently have the maximum tailcone weight installed. My understanding is if you don’t have the battery STC in front of the horizontal stab, the tailcone can take additional weight, but with the battery relocated, that weight is less. I think we’ve got all we can handle between the battery and tailcone.

>arismount: Last but not least of course I know you would never ever fly out of CG intentionally but if you ever have reason to believe you are close to your forward CG limit, do not under any circumstances land the aircraft except facing into the wind. If you know what I mean.

Words to live by………………

[john_tullamarine]

Lifeport is able to get this system certified with the simplicity of one arm station

Two immediate options ..

(a) use one arm but restrict the envelope for loading calculations to take account of the errors .. entirely commonplace solution with trimsheet loading systems, or

(b) get the system redone if the errors are other than trivial

but you have to fly all the way someplace to find that out?

phone call to someplace first to resolve the matter .. point is the problem is reasonably able to be addressed without too much drama. I well understand the practical problems if the patient is still locked in the crashed car when you launch ... guess you have to accept a number of flights with some TBA aspects for the return sector

(c) I think we’ve got all we can handle between the battery and tailcone.

not just a case of static balance .. flight loads have to be considered and ballast weight will impact on boom bending limits

[arismount]

On the Astar & EC130 the fuel tank centroid is just a little behind the mast, so if you load up with fuel it moves the CG aft. Of course the OP is probably weight limited in the first place, so this technique wouldn't be of value. Point remains though, if you ever use this to get into allowable CG, the fuel it takes to do the trick is ballast, and you can't use it...so you will be landing with a lot more on board than legal reserves.

[KrisRamJ]

As I burn fuel off in my AS355 F1 the CG moves forwards. The reason is because the rear tank is bigger than the front one and is located slightly further back.

Cheers

Kris